Generally, when the word upset is used we default to thinking of anger. Single words used to express behaviour related to anger include fume, rage, storm, rant, and rave.
However, you mention in a comment to the question that by upset you actually mean a combination of sorrow and disappointment.
Cry is certainly behaviour that could be expressed by somebody feeling sorrow and disappointment—but since you aren't using it, it seems you are looking for something else.
I could list a long series of synonyms here, but you are talking about your son not doing something you want him to do. If you are feeling "sorrow and disappointment," it's probably because you fear for the consequences of his not doing what you've asked him to do.
Based on this, I would say:
I will worry.
This is a subtle consequence to express to a child, as it describes something internal rather than observable behaviour. But based on his empathy and intuition (and if he's old enough to understand it) it might be a more honest communication.